612 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



pair of silicon carbide varistors serves for both the sidetone balancing 

 and equalizing functions, and with certain novel modifications in the 

 circuit of the set, yields essentially the same overall transmission per- 

 formance as the original design at lower cost. The advantage is retained 

 of a telephone set which from a transmission standpoint is universal in 

 its application in the telephone plant. This universality avoids additional 

 codes of set which require effort and expense to administer. Telephone 

 sets having these features are designated as 500C (manual) and 500D 

 (dial) types. The original design is known as the 500 A (manual) and 

 500B (dial types). 



GENERAL 



When the sidetone, which is the sound level in the receiver caused by 

 voltage developed in the local transmitter, becomes too high, the sub- 

 scriber subconsciously lowers his talking level thereby reducing the 

 level of outgoing transmission.^' ^ To avoid this loss and make the higher 

 efficiency provided by the new transmitter and receiver effective, a 

 reduction in sidetone must be made. When the room noise level at the 

 station is high, another undesirable effect of high sidetone occurs. This 

 is the loss in effective receiving which results from the masking of in- 

 coming speech by the high level of sidetone noise in the receiver. This 

 will be discussed in more detail later. 



In developing the original design of 500-type set, several different 

 equalizing means were considered, and the type involving a filament in 

 series with the transmitter, and a thermistor bead in shunt with the 

 receiver was selected as the most suitable with the means available at 

 that time. Reduction in sidetone was realized by the use of a special 

 balancing network which required an auto transformer. The set now 

 being produced is based on a new circuit arrangement, utilizing a pair 

 of semiconductors in the form of silicon carbide varistors, one of which 

 has a resistance in a range which required development for this specific 

 purpose. 



CHARACTERISTICS OF SILICON CARBIDE VARISTORS 



Fig. 1 shows the relation between the dc voltage and current for the 

 two varistors used in the 500C and D sets. These varistors were coded 

 312D and 312E, but for brevity in this paper, they are referred to as Vi 

 and V2 respectively. Also shown in Fig. 1 is the dc resistance of these 

 varistors as determined by the ratio of applied voltage to current. It is 



